It mostly use when you have many bad experiences recently ,
and to carry some bad luck with you .
for example : You just wake up and going go to school , but find out that your alarm was broke your almost late , and after you arrive , you see that you forgot your homework at home ...etc the things happen today are totally not in good ways than you could use the sentence 我真的是衰神上身耶 to tell others how you feel on these events happened